The invention relates to a radio receiver in which the received signal is converted by mixing into an intermediate frequency signal and/or a baseband signal. As is well-known, the purpose of radio receivers is to receive electromagnetic waves. Radio receivers are, for example, radio broadcast receivers, television receivers or radiotelephones.
The problem that a signal spectrum whose spectral components may exhibit level differences of up to 120 dB must be processed by the receiver is known to occur in radio receivers. The high-level signal components mostly result in interfering disturbances such as, for example, multiple reception through harmonic mixing and intermodulation. Such disturbances are known to be caused by the signal-dependent driving of the non-linear components located in the signal path.
The intermodulation disturbances are particularly critical since they may already occur at a relatively low disturbance signal level. Intermodulation disturbances are disturbances which are caused by at least two disturbance signals and constitute a disturbance when the frequencies of, for example, two disturbance signals with the frequencies f.sub.s1 and f.sub.s2, respectively, have such a constellation in relation to each other that one of the two conditions EQU 2f.sub.s1 -f.sub.s2 =f.sub.e or 2f.sub.s2 -f.sub.s1 =f.sub.e
is met, with f.sub.e being the frequency of the desired signal or the adjusted receiving frequency.
In this case, an "apparent" desired signal which--in the case of two disturbance signals--typically contains the modulation content of both disturbance signals, may be received on the desired frequency. Such a reception situation is then mostly interpreted by the user, for example, the radio broadcast listener, as lacking selectivity of the receiver. Another disturbing effect of the intermodulation is the interference formation with a weaker desired signal, which could be satisfactorily received, for example, without intermodulation or at low intermodulation. The danger of the disturbing effect of high-level signal components at the receiver input generally increases overproportionally with the number of high-level signal components and with their level. In most cases, intermodulation disturbances in a radio receiver are formed in the receiver stages before the channel selection, i.e., in the receiver prestage (RF) or in the mixing stage. Components which cause such disturbances are, for example, bipolar transistors, field-effect transistors and diodes; varactor diodes are also included in the intermodulation forming components.
In technical literature, the intermodulation behavior of a radio receiver is characterized by the so-called intercept point. It is the third order intercept point that is meant. The intercept point is apparent from the diagram of FIG. 1. Indicated in this diagram, on the abscissa axis are the level of the desired signal P.sub.e and also the level P.sub.s1 and P.sub.s2 of the two disturbance signals causing the intermodulation, and on the ordinate axis, as an example, the intermediate frequency signal level P.sub.if at the output of the receiver mixing stage. Curve 1 shows the output signal level P.sub.if in dependence upon the desired signal P.sub.e. Curve 2 describes the dependency of the output level P.sub.if on disturbance signal levels P.sub.s1 and P.sub.s2, which cause the intermodulation (3rd order). In the illustration of the dependency it is assumed that both axes of FIG. 1 have logarithmic graduation, that the two disturbance signal levels are, furthermore, identical and that there is also no amplification control in the signal path. It is, furthermore, assumed that the level at which a signal/disturbance ratio of 30 dB occurs at the receiver output, in relation to a predetermined desired signal modulation, is regarded as the smallest desired signal level in the diagram (zero point of the abscissa axis). The point of interseption of the two curve tangents produces a fictitious point in the diagram, the so-called intercept point, which is assocaited with a certain input level, the fictitious level of the disturbance signals forming the intermodulation and a certain fictitious IF output level. The gradients of the two curve tangents typically differ by the factor 3. In radio receivers, the intercept point is generally indicated in relation to the receiver input level (IP.sub.3).
A large level value of the intercept point is aimed at for a radio receiver. The larger this value is, the larger are the disturbance signal levels which the receiver can process without a disturbing effect through intermodulation. An increase in the intercept level of a receiver is, however, limited by economic considerations.
In order to reduce intermodulation disturbances or, more generally, interfering disturbances, it is known to control the amplification at the receiver input in dependence upon the input signal, for example, by controlling an amplification component or by controlling a damping member comprised, for example, of PIN diodes. In known radio receivers, the control signal controlling the amplification or damping is produced, for example, by rectification of the amplified intermediate frequency signal and/or by rectification of the signal before the channel selection, for example, via the output of the prestage or via the input or the output of the mixing stage.
The positive effect of such a control for decreasing interfering disturbances does, however, only occur when the component controlling the amplification or the damping is arranged before the receiver stage forming the interference, and the controlled component itself does not contribute towards the interference.
The disadvantage of the known radio receiver circuit, wherein the control signal is produced by rectification of the IF signal in the signal path of the receiving section, is that complete suppression of the demodulation of an interference signal is, in principle, not possible since the control signal required to suppress the interference cannot be produced in the control circuit by the interference signal itself. If an interference signal already produces an effective control signal, the interference signal is also demodulated and, therefore, causes a disturbance. In this case, the interfering disturbance can only be suppressed by a correspondingly strong desired signal. In a radio receiver of the described kind, the disturbance likelihood through interference may only be reduced by reducing the threshold level for signal gain/damping control.
This measure does, however, have the disadvantage that the maximum signal/noise ratio of a received desired signal that is attainable remains correspondingly low since for a desired signal, above the threshold level, the signal/noise ratio practically no longer increases with the desired signal level.
The disadvantage of the known radio receiver circuit, wherein the control signal is produced in broadband configuration by rectification of the signal before the IF selection, is that with the presence of high-level signals which are not desired signals and which produce correspondingly control signals, the entire signal composition, also the desired signals at the receiver input, are attenuated, and, more particularly, also when no disturbance would occur on account of the frequency constellation of the disturbance signals. Even one single strong disturbance signal which does not from an intermodulation product, impairs or thus stops reception of weaker desired signals.